You shouldn't make changes directly to the installed template (Classic); instead, copy the /includes/templates/classic/ folders and files to /includes/templates/your_template/ (name it as you wish, but no spaces or punctuation, and better to avoid caps). Edit /your_template/template_info.php in a text editor to fit your custom info, and follow template creation/installation instructions in the FAQ/tutorials to turn it on.

Then have fun, play with it until it breaks, and just make a new copy if the one you're working on does break. That's an important reason to keep the /classic/ files untouched.